Lateral compartment knee tibio-femoral radiographic osteoarthritis (TFROA) is associated with pain and disability, and studies of this form of knee OA are few. Recent studies find that race, pelvic anatomy and malalignment are associated with TFROA. Previous studies of knee OA have either combined medial and lateral compartment TFROA or have focused on medial compartment disease. We intend to elucidate the risk factors for and health outcomes of lateral compartment TFROA. Specifically we will (1) examine the association of lesions detected by MRI with lateral compartment TFROA; (2) examine known risk factors for medial compartment OA and their relation to prevalent and incident lateral compartment TFROA; (3) examine novel risk factors (e.g., hip and knee shape) that likely have compartment-specific effects and their relation to prevalent and incident lateral compartment TFROA; (4) examine the association of lateral compartment TFROA with incident functional impairment. We will use information collected in the Osteoarthritis Initiative (OAI) including longitudinal knee and hip x-ray and 3.0 Tesla MRI scans as well as detailed repeated risk factor, pain, and outcomes data. This proposed study will be one of the first done in a large, well-defined cohort with excellent follow-up, imaging and data collection of known and potential risk factors to focus specifically on lateral compartment TFROA.